Granular starch is found in grains, cereals or tubers of plants. The granular starch is in the form of microscopic granules, which are insoluble in water at room temperature. When an aqueous starch slurry is heated, the granules swell and eventually burst, dispersing the starch molecules into the solution. During this “gelatinization” process, there is a dramatic increase in viscosity. Because the solids level is 30-40% in a typical industrial process, the starch has to be thinned or “liquefied” so that it can be handled during the following process steps. This reduction in viscosity is generally accomplished by enzymatic degradation in a process referred to as liquefaction. During liquefaction, the long-chained starch is degraded into smaller branched and linear chains of glucose units (dextrins) by an alpha-amylase.
A conventional enzymatic liquefaction process may be carried out as a three-step hot slurry process. The slurry is heated to between 80-85° and thermostable alpha-amylase added to initiate liquefaction. The slurry is then jet-cooked at a temperature between 105-125° C. to complete gelatinization of the slurry, cooled to 60-95° C. and, generally, additional alpha-amylase is added to finalize hydrolysis. The liquefaction process is generally carried out at pH between 5 and 6. Milled and liquefied whole grains are known as mash.
During saccharification, the dextrins from the liquefaction are further hydrolyzed to produce low molecular sugars DP1-3 that can be metabolized by yeast. The hydrolysis is typically accomplished using glucoamylases, alternatively or in addition to glucoamylases, alpha-glucosidases and/or acid alpha-amylases can be used. A full saccharification step typically last up to 72 hours, however, it is common only to do a pre-saccharification of, e.g., 40-90 minutes at a temperature above 50° C., followed by a complete saccharification during fermentation in a process known as simultaneous saccharification and fermentation (SSF).
Fermentation, may be performed using a yeast, e.g., from Saccharomyces spp., which added to the mash. When the alcohol product is recovered ethanol, e.g. fuel, potable, or industrial ethanol, the fermentation is carried out, for typically 35-60 hours at a temperature of typically around 32° C. When the alcohol product is beer, the fermentation is carried out, for typically up to 8 days at a temperature of typically around 14° C.
Following fermentation, the mash may be used, e.g. as a beer, or distilled to recover ethanol. The ethanol may be used as, e.g., fuel ethanol, drinking ethanol, and/or industrial ethanol.
It will be apparent from the above discussion that the starch hydrolysis in a conventional alcohol product process is very energy consuming due to the different temperature requirements during the various steps. U.S. Pat. No. 4,316,956 provides a fermentation process for conversion of granular starch into ethanol. The European Patent EP0140410B2 provides an enzyme composition for starch hydrolysis. The object of the present invention is to provide improved processes for conversion of granular starch into alcohol products.